Reactions of Pinacols with One-Electron Oxidants

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Abstract
Oxidation of the tetraarylpinacols (Ar2COH)2, 1a−e, in which Ar = C6H5 (1a), 4-ClC6H4 (1b), 4-MeC6H4 (1c), 4-MeOC6H4 (1d) and 4-Me2NC6H4 (1e), by thianthrene cation radical (Th•+) in CH3CN and in CH2Cl2 led quantitatively to the corresponding diaryl ketones Ar2CO (2a−e), provided a sufficient amount of base, 2,6-di-tert-butyl-4-methylpyridine (DTBMP), was present to prevent presumed acid-catalyzed rearrangement. In the case of 1e, continued oxidation of 2e was also observed. Oxidation of 1a by (4-BrC6H4)3N•+SbCl6- and (4-BrC6H4)3N•+SbF6- (Ar3N•+) occurred analogously. Evidence for the catalytic, cation-radical rearrangement of 1a by Ar3N•+ (reported in earlier literature) and by Th•+ could not be found. Quantitative oxidation of 1a to 2a and of 1d to 2d was obtained also with NOBF4, again provided that sufficient DTBMP was present to prevent acid-catalyzed rearrangement. Catalytic, oxidative rearrangement of 1d at room temperature and (as reported in earlier literature) at −5 °C was not observed. Oxidation was also observed of 2,3-diphenyl-2,3-butanediol (3) to acetophenone (9) and of 1,1-dimethyl-2,2-diphenylethanediol (4) to 2a and acetone by Th•+. Oxidation of 2,3-dimethyl-2,3-butanediol (5) by Th•+ was not observed. Instead, even in the presence of DTBMP, pinacolone (10) and tetramethyloxirane (11) were formed, through, it is proposed, a mechanism involving complexation with Th•+.

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